Green antioxidant composites based on coffee by-products from the alcohol industry

Bio-based high-density polyethylene (BioHDPE)-loaded composites with different amounts of spent coffee powder (up to 30 wt.%) were successfully produced. The use of coffee residues coming from the alcoholic beverage industry in a BioHDPE matrix resulted in a remarkable improvement in the antioxidant properties of the composites. Waste spent coffee powder reduced the elastic modulus, hardness and storage modulus; the reduction was proportional to the amount of by-product considered. To improve the interaction between the highly hydrophobic BioHDPE matrix and the highly hydrophilic spent coffee powder, a copolymer – namely, poly(ethylene-graft-maleic anhydride) (PE-g-MA) – was used. PE-g-MA addition resulted in an improved interaction between the particles and poly(lactic acid), as confirmed by field emission scanning electron microscopy. On the other hand, the presence of the lignocellulosic filler resulted in a low load transfer between the two components, which led to a decrease in tensile strength. The introduction of coffee powder resulted in an improvement in thermal stability, as confirmed by thermogravimetry and differential scanning calorimetry. Finally, antioxidant activity was also observed in the composites, wherein a 27% reduction in 2,2-diphenyl-1-picrylhydrazyl was reported for the composite with the highest coffee content. The presence of antioxidant substances (coffee oil) was confirmed by Fourier transform infrared spectroscopy.